Purification systems are widely used in a variety of industries for removing impurities from a stream of liquid or gaseous material. Of particular interest to the present application is a purification system for the plastics industry and, in particular, the thermoplastic waste recovery industry wherein it is a desireable goal to recover a purified thermoplastic material from scrap or waste thermoplastic material. Often such waste thermoplastic material contains impurities such as bits of metal or soil or other organic or inorganic debris which must be removed in order to produce the purified thermoplastic material.
A problem is created when the recovered material must have a particularly high degree of purity in order to be of commercial value. One such material is polypropylene. Examples of other types of thermoplastic materials are nylon, PVC and polyester.
Typically, purified polypropylene reclaimed from distressed, or waste, polypropylene is mixed with virgin polypropylene. The mixture may then be further processed to produce fibers or filaments having desireable stretch and other characteristics. If, however, the reclaimed polypropylene is of less than a given degree of purity, the reclaimed material when added to virgin polypropylene will not yield a fiber having the desired characteristics.
A process for reclaiming polypropylene typically includes heating the distressed material to above the melting point of polypropylene (approximately 340.degree. F.) and then extruding the melted material under pressure through a filter or filters to remove impurities. The filtered material may then be passed through a pelletizer to produce pellets of essentially pure polypropylene. Alternatively, the filtered material may be passed through other devices to produce, for example, filaments, sheets or films of polypropylene.
As may be appreciated, the purity of such pellets is to a large degree dependent on the effectiveness of the filter or filters. If the filters become clogged with filtrate, such as inorganic matter or gel, the flow of polypropylene from the extruder is reduced, resulting in an increase in pressure. Such an increase in pressure is often detrimental in that at a certain critical pressure point some of the filtrate may be forced to pass through the filter, resulting in the production of polypropylene pellets of less than the desired grade of purity. Thus, it is essential that the filter or filters be removed and replaced with clean filters at or just before the critical pressure point in order to produce purified pellets in a consistent and continuous manner.
It is therefore an object of the present invention to provide a purification system which allows for the consistent and continuous production of a purified material.
It is a further object of the present invention to provide a purification system wherein an increase in pressure due to the clogging of a filter with impurities is detected such that the clogged filter may be replaced with a clean filter.
It is a further object of the present invention to provide a purification system wherein an increase in pressure due to the clogging of the filter is detected such that the clogged filter may be replaced before the filtrate trapped therein is forced through the filter due to the increase in pressure beyond the critical pressure point.
It is a further object of the present invention to provide a purification system for producing in a consistent and continuous manner thermoplastic material, such as polypropylene, having a high degree of purity.
It is a still further object of the present invention to provide a method of determining the optimum time at which a filter within a purification system need be replaced, the method of determining being accomplished in part by monitoring an increase in pressure within the system due to the clogging of the filter with impurities.